As the need for mobile devices, electric cars and storage of energy has grown, there is an increased need for the development of battery technology. Lithium batteries have done well in very many applications, especially due to good energy density and recharging possibilities, compared e.g. to traditional Ni—Cd and Ni—Mn batteries.
The lithium battery technology is based on a positive cathode, in which the active material is, for example, transition metal oxide, and on a carbon-based negative anode. Microporous polymer separator is used between the anode and cathode to prevent the contact of the anode and cathode, but nevertheless allowing the movement of ions through the separator film. In addition to ion permeability, the separator film also has to have good mechanical strength and long-term resistance against heat and chemicals.
A problem with the use of polymer-based microporous films is their temperature resistance, which may be limited to below 150°, in which case a high temperature can cause a short circuit and the electrolyte to flare up and generate a fire in problem situations. Safety is important in all operating conditions, including abnormal conditions, such as crashes. It is also in this case important to try to keep the reliability of the operation of batteries as good as possible. In addition the shrinking of polymeric films during the use e.g. as the temperature rises, chemical stability and the ability of the films to bind liquid electrolyte may restrict the use of polymer membranes. It has been tried to improve the characteristics of polymer films with many different methods, examples of which are disclosed next.
The characteristics of a separator film can be boosted by coating the surface of the electrode with a combination of polymer and inorganic material, i.e. micro composite material. The manufacture is carried out by mixing an inorganic filler with polymer and solvent, by coating an electrode with this solution and by letting the solvent to exit so that a microporous mixture of polymer and inorganic material is obtained. Because the separator is not entirely made of inorganic material, its insulating capacity does not necessarily correspond to a uniform layer of inorganic material.
Also fully inorganic ceramic separator films reinforced with small amounts of binding agent have been used in lithium batteries. Their advantage is that, among other things, they have a very good thermal and chemical stability and the ability to bind liquid electrolyte. On the other hand, their use is restricted by their poor treatability, for example, in rolling and installations.
In order to enhance the reliability of separator films, their stability has been improved, for example, by coating one or both sides of the separator film with a ceramic coating in order to obtain strength, insulating size, and temperature resistance. Manufacturing ceramic coatings e.g. with different wet processes does not necessarily provide the lithium batteries with the required homogeneity. The control of pore size distribution is difficult and, for example, the production of very fine-grained pore networks may be impossible. The adherence in the polymer separator films is also not necessarily sufficient. The thickness of this type of film is often several micrometres, which in its part impedes the permeability of ions and reduces the relative amount of active material and the ability of the lithium batteries to store energy.